As it is known, optimization of connectivity to the Internet of an Internet Service Provider (ISP) is strongly related to the nature and topology of the Internet as a whole. In fact, the Internet is a combination of interconnected component networks that share the same addressing structure, a common routing vision and a name-assignment system. At present, this combination contains more than 20,000 networks or administrative domains, commonly referred to as Autonomous Systems (ASes), and this number continues to increase. An AS identifies a network managed by a single ISP, and hence these two terms are generally regarded as synonyms.
In this context, no ISP can operate in complete isolation from the others, for obvious limitations in the geographic extension of its own networks, the points of presence, and overall capacities of the Internet backbones. On the other hand, every customer of an Internet Connectivity Service provided by a given ISP expects complete and total accessibility to all destinations present on the Internet.
In order to guarantee an adequate service to its customers, each ISP, also in view of the growing competition on prices, performance and reliability, must constantly update and improve its own interconnections to the Internet, establishing and developing suitable relationships with other ISPs. These relationships can be equal relationships (bilateral or multilateral), commonly referred to as peering relationships, or customer/provider relationships, commonly referred to as transit relationships, thus giving rise to various interconnection typologies between ISPs characterized by different commercial and technical properties.
A peering relationship can be further classified according to the number of ISPs that interconnect to each other, and in particular into the following two classes: bilateral or private peering and multilateral or public peering. Bilateral or private peering represents a technical/commercial relationship between two ISPs that reciprocally guarantee connectivity to all of their customers. This relationship constitutes the execution of an equal agreement between two ISPs that exchange traffic between their networks, normally without any form of economic payment. An agreement of this type is only feasible if both of the ISPs receive the same level of benefit. Instead, in the case of multilateral or public relationships between ISPs, peering is achieved at public interchange points, commonly referred to as Internet Exchange Points (IXP), which allow the interconnection of the ASes involved via a shared level-2 device (L2 switch) or via a route-server that distributes traffic routing information to the involved ASes.
FIG. 1 shows an example of a peering relationship: AS1, AS2, and AS3, each provided with a respective routing table RT1, RT2, and RT3, reciprocally exchange routing information related to their own clients (identified by circles), consequently allowing the transit of traffic in the opposite direction. AS2 does not propagate to AS1 routing information regarding the customers of AS3 and vice versa, in this way preventing the transit of traffic from AS1 to AS3 over its own network.
In the case of customer/provider relationships, there is a transit service to the Internet, that consists in a technical/commercial relationship in which an ISP, commonly referred to as Upstream Provider, provides access to all of the destinations contained in its own routing tables to another ISP, commonly referred to as Customer ISP or AS, with an associated payment flow from the Customer ISP to the Upstream Provider. An ISP can assume the role of customer for one or more Upstream Providers, and this case is commonly referred to as Multi-homing.
FIG. 2 shows an example of a transit relationship: AS1 and AS3 are customers of AS2 and use the network or backbone infrastructure of the latter to exchange traffic between their customers.
The routing over different domains on the Internet (also referred as inter-domain routing) is performed through a protocol commonly known as Border Gateway Protocol (BGP). For a general discussion of the characteristics and the application of the BGP protocol reference can be made to “A Border Gateway Protocol 4(BGP-4)” by Y. Rekhter and T. Li, RFC 1771, T. J. Watson Research Center, Cisco, March 1995.
BGP represents the universally used protocol to allow the accessibility between different ISPs through the exchange of all information needed to route the traffic between the ASes building the Internet. The BGP protocol allows each AS to adopt its own policy in selecting the paths and propagating the reachability information towards the other networks. The routing information is contained in special routing tables, commonly known as BGP tables, which represent ISPs' intra-domain routing information and are built from the BGP messages exchanged between the ISPs at the interconnection points.
With the big and fast expansion of the Internet, in modern telecommunications networks the issues related to peering and, more in general, to the interconnection between different domains have become increasingly important, especially for ISPs that have Internet access services as an element of their core business. Furthermore, the context is highly dynamic: in fact, if on the one hand the ISPs that make up the core of the Internet network are well known and relatively stable (the so-called top-level or Tier 1 ISPs, such as those defined by http://en.wikipedia.org/wiki/Tier—1_carrier#List_of_Tier—1_ISPs, in alphabetic order: AOL Transit Data Network, AT&T, Global Crossing (GX), Level 3, MCI, Nippon Telegraph and Telephone (Verio), Qwest, SAVVIS, and Sprint Nextel Corporation), on the other hand a whole series of second and third level ISPs (defined as Tier 2 or Tier 3) exists that, although characterized by less extensive and capillary networks than those of Tier 1, are nevertheless extremely numerous and offer new opportunities of interconnection and business relationships.
The consequence of this is that while in the past each ISP sought to extend and improve its own interconnection to the Internet by establishing the greatest possible number of peering relationships with subjects of equal standing, in today's ISPs (above all for those classified as Tier 2), awareness and need exist to build and maintain a base that is solid, but limited in the number of peering partners, partnered by just a few, highly reliable Upstream Providers.
The technical and commercial value of interconnections between ISPs is also highlighted by the fact that ad hoc workgroups or departments have been set up within ISPs' organizational structures to deal solely with peering management (technical aspects of networks, provisioning and marketing). Often, the activities of these groups, and in particular the choice of optimal interconnections to improve Internet connectivity, are only guided and supported by heuristic methods based on simple operating measurements or the experience and relational contacts of individuals. For a discussion regarding peering strategies reference may be made to The Art of Peering: The Peering Playbook, at http://www.xchangepoint.net/info/wp20020625.pdf.